Modern steelmaking industries need to monitor the process characteristics of the steelmaking process due, in part, to the unknown composition of scrap steel that can give rise to inaccuracies in achieving the desired end point concentration of carbon and melt temperature in the final steel product. Further, real time process monitoring in the steelmaking process is required to ensure safety, minimize pollutant emissions, and maximize productivity and energy efficiency—factors the steelmaking industry is sensitive to, due to increasing environmental regulations and ever greater competition within the industry.
Steelmaking technologies of today generally use either basic oxygen furnaces (BOFs) or electric arc furnaces (EAFs). EAFs are enjoying an increase in market share due, in part, to the EAFs ability to use 100% recycled scrap metal which, in turn, results in lower energy requirements per unit production (a large part of the energy savings for EAFs arises from avoidance of mining, smelting, and refining the raw ore). Additional savings can occur since the primary energy source for EAFs is electrical energy, rather than fossil fuels—particularly desirable given that the steelmaking industry is a source of both greenhouse gases (mainly CO2), as well as pollutants such as CO, NOx and other noxious substances, such as dioxins, hydrocarbons, and other particulates.
Notwithstanding the above benefits of EAFs current EAFs are often limited to energy efficiencies on the order of about 50–60%. Further efficiency to the steelmaking process can be achieved by improved process control during the combustion application, particularly by using non-intrusive, real time measurements of selected off-gases and temperature produced during the combustion application. For example, reducing CO to 1–2% from current industry levels in the 10–30% range can result in substantial energy savings for large EAF operations. CO provides a good empirical measure of chemical energy losses while exhaust gas temperature allows a reasonable estimate of thermal energy losses.
It can be appreciated that the requirements for a sensor to measure off-gas exhaust from an EAF is primarily driven by considerations of the harsh environment that exists in the exhaust duct where the off-gas sensor is located. Exhaust temperatures can range from about 1000 K to about 2000 K. Moreover, duct gases can have high dust concentrations that can interfere with the operation of the sensor. Further, chunks of molten slag can occasionally spew up into the duct and interfere with or damage the sensors.
Many commercial steel mills use extractive techniques to obtain a sample of off-gas from the exhaust. The extracted gas is cooled then analyzed using commercially available mass spectrometry or non-dispersive infrared absorption methods or chemical cells. It can be appreciated, however, that the steps required to obtain a sample of the off-gas from extractive techniques can result in time delays in acquiring the data. By contrast, a process control that uses real time sensors can obtain selective measurements of the off-gas constituents and provide adjustment of the inputs to a furnace (such as oxygen, fuel, electric current, etc.) on a continuous feedback loop.
Due to the harsh environment, temperature measurements are generally not available in the EAF exhaust duct because thermocouples are unable to withstand the demanding conditions.
Optical techniques, for example, can provide a non-intrusive sensor for measuring real time composition of the off-gas and temperature in an exhaust duct. The non-intrusive nature provides numerous operation and maintenance advantages in the harsh exhaust duct environment. Moreover, optical techniques offer the benefit of providing a line average concentration and temperature measurement, rather than a point measurement, which can provide more accurate and reliable approximation of average conditions with an exhaust duct of a furnace. Optical techniques generally utilize a laser beam passing straight across an exhaust duct.
One example of an optical based method and apparatus for off-gas composition sensing is disclosed in the U.S. Pat. No. 5,984,998. This patent discloses transmitting a tunable diode laser with wavelengths in the mid-infrared (mid-IR) region through the off-gas produced by a steelmaking furnace, and measuring the transmitted laser beam to produce a signal based on the wavelength absorption properties of the different off-gases. This measurement provides measurements of the gaseous constituents of the off-gas. Mid-IR diode lasers provide good sensitivity for certain molecules of interest in the off-gas, particularly, CO2, CO, and H2O.
Mid-IR laser systems, however, have certain practical limitations, particularly when operating beyond the 3.0 μm wavelengths into the mid-IR range. For example, a Pb-salt diode laser operates significantly below room temperature, necessitating cryogenic cooling. This adds to the complexity and cost of a steelmaking process control system.
Other problems using a mid-IR based laser diode sensor include signal saturation during high emission portions of the steelmaking process. Signal saturation can result in loss of process information during times of high emissions. Further, mid-IR light does not propagate readily through available fibre optics. Accordingly, the sensor should be located near the harsh environment of the exhaust duct of the furnace. This can result in a need to design special protective equipment such as water-cooling jackets and airtight seals.
In addition, mid-IR systems use mirrors to project and align the laser beam from the instrument through the desired measurement location in the exhaust duct. Ambient dust can be a problem on the electrical motors necessary to control the mirrors.